Oral squamous cell carcinoma (SCC) is the sixth most frequent cancer worldwide with an estimated 30,000 new cases and 8,000 deaths reported in the United States each year. Many oral SCC lines are immortal in vitro, suggesting that these cells have a mechanism for maintaining chromosomal integrity. The ends of human chromosomes (telomeres) lose up to 200 base pairs of DNA per cell division due to the inability of DNA polymerase to completely replicate the chromosomal ends. Chromosomal shortening ultimately leads to senescence and cell death in normal cells. Telomerase is a ribonucleoprotein complex that synthesizes telomeric DNA onto chromosomes using its RNA component as a template. Our preliminary data have shown that all SCC lines tested expressed high levels of telomerase activity and interestingly, that induction of a key cell cycle protein Rb (retinoblastoma) downregulates telomerase activity. Based on our preliminary studies, we hypothesize that telomerase activation in oral cancer cells is regulated via cell cycle dependent phosphorylation of Rb and activation of E2F-1 transcription factors. In specific aim 1, we propose to determine the role of Rb in regulating telomerase activity in oral cancer cells. In specific aim 2, we will determine how Rb phosphorylation by cyclin dependent kinases regulates telomerase activity in oral cancer cells. In specific aim 3 of this proposal, we will characterize the functional domains of the transcriptional factor E2F-1 and its regulation of the telomerase promoter. These studies may lead to the development of telomerase inhibitors for oral cancer which do not affect non-cycling cells.